The Kelvin-Helmholtz instability of evaporating thin falling film flow in vertical tube is studied by method of energy analysis. Based on the rules that the interfacial capillary waves come from the balance of works done by inertial force, surface tension on phase-change interface, and also capillary force on tube wall, the stability behaviors of falling film with different Reynolds number and different perturbation wavelength are explored in detail. The analysis indicates that the main reason of film breakup by increasing tube wall heat flux is that, the stability effect of capillary adsorbability on tube wall is weakened as surface tension waving is enhanced by improving tube wall temperature.